The present invention relates to a capacitive load driving unit integrally incorporating a capacitive load such as a print head for an ink-jet printer and a drive circuit for the capacitive load, and a method and apparatus for inspecting the unit.
The print head of a typical ink-jet printer has a plurality of ink chambers arranged in a line and partitioned by a plurality of electrostrictive members. Each of the ink chambers jets or ejects drops of ink from an ink-jet nozzle thereof upon a change in pressure caused when a corresponding electrostrictive member deforms according to a voltage applied to a pair of electrodes formed on both sides of the electro-strictive member. The pair of electrodes and the electro-strictive member constitute a capacitive load.
FIG. 21 shows an example of a drive circuit for driving such a capacitive load. This drive circuit includes a pair of transistors Tr1 and Tr2 and a level converter L. A capacitive load H is connected between a ground terminal GND and output terminal OUT of the drive circuit. The transistor Tr1 is connected between a source voltage terminal VD and the output terminal OUT. The transistor Tr2 is connected between the output terminal OUT and the ground terminal GND. The level converter L generates voltage signals for alternately turning on these transistors Tr1 and Tr2. The voltage across the capacitive load H is set to a level equal to that of the source voltage terminal VD by a charge current flowing from the source voltage terminal VD to the capacitive load H through the transistor Tr1 when the transistor Tr1 is turned on. In addition, the voltage across the capacitive load H is set to a level equal to that of the ground terminal GND by a discharge current flowing from the capacitive load H to the ground terminal GND through the transistor Tr2 when the transistor Tr2 is turned on.
When the above print head and drive circuit are to be manufactured as a single head unit, the connection state between the head and drive circuit is inspected or checked to determine whether the head unit is non defective or defective. However, the print head and the drive circuit are interconnected by a large number of wiring lines such as about 100 to 3000 wiring lines which are very thin and arranged at a small pitch of about 50 to 200 xcexcm. For these reasons, it is nearly impossible that a probe for picking up a current or voltage is sequentially set to be in contact with the wiring lines when the print head is driven.
Under the circumstances, Jpn. Pat. Appln. KOKAI Publication No. 10-86358 discloses a head unit 4 which is, as shown in FIG. 22, comprised of a unit interface 1, drive circuit 2, and print head 3, and a solder point 7. In the head unit 4, the solder point 7 is formed as a melt coupler between a drive circuit ground line 5 and a head ground line 6. A waveform of a current flowing from the print head 3 to the head ground line 6 is detected with a current probe 8 while the solder point 7 is set in a disconnected state, and this current waveform is converted into a voltage waveform by a current-voltage converter. The voltage waveform is integrated by an integrating device and supplied to a waveform recorder. The waveform recording unit records this waveform. The recorded waveform is compared with a normal waveform to determine whether the print head unit 4 is nondefective or defective. After the inspection, the solder point 7 is melted for short-circuiting so as to prevent currents for charging and discharging the print head 3 from flowing outside the head unit 4.
A power source voltage is applied from an external power source to a head unit via a relatively long cable. In a case where the print head is driven at a high speed, the inductance component of the cable greatly influences the current-supply ability of the external power source, so that current supply cannot follow the charging cycle of the capacitive load. To cope with the problem, the head unit 4 may have a bypass capacitor which is connected between the power lines 5 and 6 and located near the drive circuit 2 to be subjected to almost no influence of the inductance component of the cable. The capacitor is charged by the drive voltage applied from the external power source to reserve a current required by the drive circuit 2 for charging the capacitive load.
When the capacitor is incorporated in the head unit 4 of the reference and the waveform of a current flowing from the print head 3 to the head ground line 6 is detected as described above, the current waveform cannot be correctly detected owing to the influence of the capacitor. As a result, whether the head unit is nondefective or defective cannot be accurately determined.
It is an object of the present invention to provide a capacitive load driving unit, an inspection method and an inspection apparatus, for the unit, which permit a defect inspection to be performed without being influenced by a capacitor for reserving a current for charging a capacitive load.
According to the present invention, there is provided a capacitive load driving unit which comprises a plurality of capacitive loads, a drive circuit for charging and discharging the capacitive loads, a pair of power lines for supplying a drive voltage from an external power source to the drive circuit, and a current reserve circuit for reserving a current required by the drive circuit for charging the capacitive loads, wherein the current reserve circuit includes a- capacitor that is charged by the drive voltage from the power lines, and a coupler section for coupling the capacitor between the power lines except for when a defect inspection is conducted.
Further, according to the present invention, there is provided an inspection method for a capacitive load driving unit which comprises a plurality of capacitive loads, a drive circuit for charging and discharging the capacitive loads, a pair of power lines for supplying a drive voltage from an external power source to the drive circuit, and a current reserve circuit for reserving a current required by the drive circuit for charging the capacitive loads, wherein the current reserve circuit includes a capacitor that is charged by the drive voltage from the power lines, and a coupler section for coupling the capacitor between the power lines except for when a defect inspection is conducted, the method comprising the steps of controlling the drive circuit to sequentially drive the plurality of capacitive loads in a state where the capacitor is disconnected from the power lines; and detecting an electric change that occurs on the power line upon driving each of the capacitive loads to determine whether or not a defect is present.
Moreover, according to the present invention, there is provided an inspection apparatus for a capacitive load driving unit which comprises a plurality of capacitive loads, a drive circuit for charging and discharging the capacitive loads, a pair of power lines for supplying a drive voltage from an external power source to the drive circuit, and a current reserve circuit for reserving a current required by the drive circuit for charging the capacitive loads, wherein the current reserve circuit includes a capacitor that is charged by the drive voltage from the power lines, and a coupler section for coupling the capacitor between the power lines except for when a defect inspection is conducted, the apparatus comprising a control circuit for controlling the drive circuit to sequentially drive the plurality of capacitive loads in a state where the capacitor is disconnected from the power lines, and a detecting circuit for detecting an electric change that occurs on the power line upon driving each of the capacitive loads to determine whether or not a defect is present.
In the capacitive load driving unit, the inspection method, and the inspection apparatus, the coupler section causes the capacitor for reserving a charging current for the capacitive loads to be disconnected from the power lines in a defect inspection. Therefore, the defect inspection can be performed without being influenced by the capacitor.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.